1. Field of the Invention
The present invention relates to a laser driving method and apparatus for making a laser emitting device emit a laser beam controlled according to a control signal and to an image-forming apparatus of an electrophotographic method making use of the laser driving apparatus for emitting the laser beam by the laser driving method.
2. Related Background Art
The conventional image-forming apparatus of the electrophotographic method such as laser printers or the like are provided with the laser driving apparatus and this laser driving apparatus emits the laser beam according to image data. The laser driving apparatus of this type is provided, for example, with a semiconductor laser as laser emitting means and a pulse current is supplied to this semiconductor laser, whereupon the semiconductor laser emits the laser beam in a light amount according to the current.
At this time, the laser beam emitted from the semiconductor laser is controlled according to the image data and this laser beam is deflected by a rotating polygon mirror or the like to scan in the main scanning direction. At the same time as it, a photosensitive drum as latent image carrying means is rotated to move a peripheral surface thereof at an exposure position in the sub-scanning direction and the peripheral surface of the photosensitive drum moving in the sub-scanning direction is charged by a charging charger.
The peripheral surface of the photosensitive drum rotating in the sub-scanning direction as being charged is exposed to and scanned with the laser beam thus deflected to scan, so as to form an electrostatic latent image thereon and this electrostatic latent image is developed with toner by a developing device as latent image developing means. The toner on the peripheral surface of the photosensitive drum, thus developed, is transferred onto a recording medium by a transfer charger as toner transferring means and the toner transferred onto the recording medium is fixed by a fixing device as toner fixing means.
Some of the image-forming apparatus described above are arranged to always supply a current near a laser emission threshold to the semiconductor laser and supply a pulse current or a bias current controlled according to the image data when necessary. Since a photodiode to function as laser monitor means is usually integrated in the semiconductor laser, it is also normal practice to monitor the amount of emitted light from the semiconductor laser by this photodiode and maintain the laser light amount constant by adjusting the bias current or the pulse current.
A conventional example of the image-forming apparatus as described above will be explained referring to FIG. 1 to FIG. 3. FIG. 1 is a schematic block diagram to show the main part of the image-forming apparatus, FIG. 2 is a schematic plan view to show part of an optical system of the image-forming apparatus, and FIG. 3 is a time chart to show the relation among various signals.
First, the image-forming apparatus 1 of this conventional example has a laser device 2, as illustrated in FIG. 1, and this laser device 2 is constructed in an integral form comprised of a semiconductor laser 3 as laser emitting means and a photodiode 4 as laser monitor means. The semiconductor laser 3 emits the laser beam in response to supply of current, and the photodiode 4 monitors the laser beam emitted from the semiconductor laser 3 and outputs a current signal according to the light amount of the laser beam.
Connected to the laser device 2 are a power supply 5 of the main body and a laser driver 6, and this laser driver 6 incorporates a pulse current source 7 being pulse supply means, a bias current source 8 being bias supply means, and an APC (Automatic Power Control) circuit 9 being current adjusting means.
The two current sources 7, 8 are connected in parallel to one semiconductor laser 3, the pulse current source 7 being connected directly to the semiconductor laser 3 while the bias current source 8 being connected via a high-speed analog switch 10, such as a CMOS (Complementary Metal Oxide Semiconductor) or the like, to the semiconductor laser 3.
The pulse current source 7 generates a pulse current and supply it to the semiconductor laser 3, whereas the bias current source 8 supplies a bias current controlled by on/off of the analog switch 10 according to a control signal supplied from the outside, to the semiconductor laser 3 when necessary.
The APC circuit 9 is connected to the photodiode 4 and to the pulse current source 7 and adjusts the pulse current of the pulse current source 7 according to the result of detection by the photodiode 4. More specifically, the APC circuit 9 is comprised of a current-voltage converter 11, a sample hold circuit 12, a constant voltage supply 13, a comparator 14, etc., and converts a current sinal outputted from the photodiode 4 to a voltage signal by the current-voltage converter 11.
The sample hold circuit 12 samples and holds the voltage signal from the current-voltage converter 11 at predetermined timing and the comparator 14 compares the hold voltage of the sample hold circuit 12 with a reference voltage of the constant voltage supply 13 to increase or decrease the pulse current generated from the pulse current source 7.
An image processing circuit 15 being signal input means is connected to the analog switch 10, which is connected to the bias current source 8, and a system controller 16 is connected to this image processing circuit 15. The image processing circuit 15 switches the analog switch 10 on and off, using image data supplied as a control signal from the outside, and the system controller 16 systematically controls the circuits including the image processing circuit 15 etc.
A reflective surface of a polygon mirror 21 being beam deflecting means is located through a collimator lens 20 on the optical axis of the semiconductor laser 3 of the laser device 2, as illustrated in FIG. 2, and the peripheral surface of a photosensitive drum 23 being the latent image carrying means is located, for example, through a correction optical system 22 of an fxcex8 lens or the like on the reflected light path of this polygon mirror 21.
The polygon mirror 21 is rotatably supported by a scanner motor (not illustrated) and deflects the laser beam emitted from the semiconductor laser 3 to effect scanning in the main scanning direction. The photosensitive drum 23 is rotatably supported by a drum driving mechanism (not illustrated) being sub-scanning means and the peripheral surface thereof exposed to and scanned with the laser beam is relatively moved in the sub-scanning direction.
A BD (Beam Detect) sensor 24 being beam detecting means is placed at a forward position in the main scanning direction with respect to the photosensitive drum 23 in the scanning range of the polygon mirror 21 and this BD sensor 24 detects the laser beam under deflection scanning by the polygon mirror 21 immediately before irradiation of the photosensitive drum 23 therewith.
The BD sensor 24 is also connected, for example, through an amplifier (not illustrated) or the like to the system controller 16 and this system controller 16 controls the operation of the image processing circuit 15, the APC circuit 9, etc. according to the timing of detection of the laser beam by the BD sensor 24 and the like.
In that case, as illustrated in (b) in FIG. 3, during the time period xe2x80x9cT1 to T2xe2x80x9d of initial setting, the analog switch 10 is kept continuously on by the data signal being a control signal of the image processing circuit 15, to make the bias current source 8 and the pulse current source 7 supply the respective currents to the semiconductor laser 3, thereby making the semiconductor laser 3 continuously emit the laser beam.
At this time, because the voltage according to the emitted light amount of the laser beam continuously emitted from the semiconductor laser 3 is sampled by the sample hold circuit 12 in the APC circuit 9 as illustrated in (c) in FIG. 3, the pulse current IT of the pulse current source 7 is initially set according to the voltage sampled by this sample hold circuit 12 as illustrated in (a) in FIG. 3.
After completion of the initial setting as described above, the deflection scanning with the laser beam is started with rotational driving of the polygon mirror 21 and the pulse current source 7 is finely adjusted every main scanning line by the APC circuit 9 during the time period xe2x80x9cT3 to T4xe2x80x9d immediately before the laser beam under the deflection scanning is detected by the BD sensor 24.
During the time period xe2x80x9cT5 to T6xe2x80x9d immediately after it, the laser beam under the deflection scanning is detected by the BD sensor 24 and after a lapse of a predetermined time from this beam detection, the laser beam starts exposure and scanning of the image area of the photosensitive drum 23. In this case, the image processing circuit 15 controls on/off of the analog switch 10 according to the image data to make the bias current source 8 and the pulse current source 7 supply the bias current and the pulse current to the semiconductor laser 3 so as to make the semiconductor laser 3 emit the laser beam controlled according to the image data.
Although omitted from the illustration and description because of the generally known structure, various devices, including a charging charger as carrier charging means, a developing device as latent image developing means, a transfer charger as toner transferring means, and so on, in addition to the laser scanning mechanism described above, are opposed to the peripheral surface of the photosensitive drum 23 and a conveyance passage of a print sheet being a recording medium is also created in a gap between the transfer charger and the photosensitive drum 23.
The image-forming apparatus 1 of the above-stated structure can form an image by the electrophotographic method.
Since the image processing circuit 15 controls on/off of the analog switch 10 of the bias current source 8 according to the image data while the pulse current source 7 supplies the pulse current to the semiconductor laser 3, this causes the semiconductor laser 3 to emit the laser beam controlled according to the image data.
The laser beam thus emitted according to the image data from the semiconductor laser 3 is deflected by the rotating polygon mirror 21 to scan in the main scanning direction and irradiate the peripheral surface of the photosensitive drum 23 rotating in the sub-scanning direction, whereby an electrostatic latent image is formed thereon in the form of a lot of main scanning lines.
At this time, the laser beam under the deflection scanning is detected by the BD sensor 24 immediately before the irradiation of the photosensitive drum 23. Emission of the laser beam according to the image data is started at a predetermined time after the detection of the beam by the BD sensor 24, whereby start positions of the many main scanning lines continuous in the sub-scanning direction can be aligned.
Since the laser beam needs to be detected by the BD sensor 24 prior to the image scanning as described above, the semiconductor laser 3 is driven by the system controller 16 and the image processing circuit 15 at the timing when the laser beam under the deflection scanning irradiates the BD sensor 24.
The semiconductor laser 3 is also driven for the APC operation immediately before the BD sensor 24 detects the laser beam. The emitted light amount of the laser beam is detected by the photodiode 4 and the pulse current from the pulse current source 7 is adjusted by the APC circuit 9 of the laser driver 6.
Since in the above-stated image-forming apparatus 1 the pulse current is adjusted so as to keep constant the emitted light amount of the laser beam emitted from the semiconductor laser 3, the image can be formed with good quality. Further, because the bias current controlled according to the image data is supplied while the pulse current is supplied to the semiconductor laser 3, the image can be formed at high speed with the semiconductor laser 3 being driven in good response. The image-forming apparatus described above is arranged to adjust the pulse current, but it can also be contemplated, conversely, that the bias current is adjusted to keep the emitted light amount of the laser beam constant.
The image-forming apparatus 1 described above is arranged to enhance the response of the semiconductor laser 3 so as to perform the image formation at high speed by supplying the bias current controlled according to the image data to the semiconductor laser 3 under supply of the pulse current, and it is now under research to simultaneously form plural main scanning lines using a plurality of semiconductor lasers in the laser device in order to further increase the speed of image formation of the image-forming apparatus.
In general, even if the laser device is comprised of a plurality of semiconductor lasers as described above, it is preferable to use only one photodiode in terms of productivity and scale reduction. For detecting amounts of emitted light from the respective semiconductor lasers in order to adjust the pulse current or the bias current, one photodiode is arranged to detect each of the emitted light amounts while the semiconductor lasers are driven in order.
However, where the current near the laser emission threshold is always supplied to the plural semiconductor lasers by use of the pulse current and the bias current in order to enhance the response as described above, this current near the threshold sometimes causes a semiconductor laser in a non-driven state to emit a small amount of light, which does not allow accurate detection of the emitted light amount of the semiconductor laser in a driven state. This results in failing to properly adjust the bias currents of the plural semiconductor lasers, thereby degrading the quality of the image formed by the image-forming apparatus.
The present invention has been accomplished in view of the above-stated issue and an object of the present invention is to provide a laser driving method and apparatus capable of accurately detecting and correcting emitted light amounts of plural laser emitting means under supply of the pulse current and bias current by one laser monitor means, and also to provide an image-forming apparatus capable of forming an image with high quality.
A laser driving method of the present invention is a laser driving method comprising steps of always generating a first current to be supplied to each of plural laser emitting means, generating plural second currents controlled according to a control signal supplied from the outside, when necessary, supplying these plural second currents to the respective laser emitting means under supply of the respective first currents to make the plural laser emitting means emit respective laser beams, making these plural laser emitting means emit the laser beams in time division at predetermined timing and making one laser monitor means detect each of emitted light amounts, and adjusting at least one of the first current and the second current supplied to each of the plural laser emitting means according to these light amounts detected,
wherein supply of the current to the laser emitting means not driven is stopped when the plural laser emitting means are adjusted in light amount in time division.
Therefore, the current is not supplied to the laser emitting means not driven when the plural laser emitting means are adjusted in light amount in time division and one laser monitor means detects the laser light amounts, so that when only one of the plural laser emitting means is driven for the measurement of light amount, the laser emitting means not driven are prevented from emitting a small amount of light.
A laser driving apparatus of the present invention comprises: signal input means through which a control signal is supplied from the outside; a plurality of second current supply means for generating a second current controlled according to the control signal supplied from the outside through said signal input means, when necessary; a plurality of first current supply means for always generating a first current; a plurality of laser emitting means for individually emitting a laser beam according to the second current and the first current supplied respectively from either of said plurality of second current supply means and from either of said plurality of first current supply means; driving means for making said plurality of second current supply means generate the predetermined second current at predetermined timing to make said plurality of laser emitting means emit respective laser beams in time division; one laser monitor means for monitoring each of the laser beams which said driving means makes said plurality of laser emitting means emit in time division, to detect emitted light amounts of the respective laser beams; current adjusting means for individually adjusting an output current from at least either said first current supply means or said second current supply means according to a plurality of detection results of said laser monitor means; and current control means for stopping supply of the current to the laser emitting means not driven when said driving means drives said plurality of laser emitting means in time division.
When the control signal is supplied from the outside into the signal input means, the plurality of second current supply means generate the second currents controlled according to this control signal, when necessary. At this time the plurality of first current supply means always generate the first currents and thus the plural laser emitting means emit the respective laser beams according to the second current and the first current supplied to each means from the plurality of second current supply means and from the plurality of first current supply means. When the driving means makes the plurality of second current supply means generate the predetermined second currents at predetermined timing to make the plural laser emitting means emit the laser beams in time division, each of the laser beams emitted in time division from the plural laser emitting means is monitored by one laser monitor means to detect each emitted light amount, and each of the output currents from the first current supply means and second current supply means is adjusted by the current adjusting means according to the plural detection results. Since the supply of current to the laser emitting means not driven is stopped by the current control means when the plural laser emitting means are driven in time division and their laser light amounts are detected by one laser monitor means as described above, the laser emitting means not driven, however, are prevented from emitting a small amount of light when only one of the plural laser emitting means is driven for the measurement of light amount.
The present invention also provides the above-stated laser driving apparatus wherein said current control means comprises: at least one third current supply means for generating a current for canceling out a current near a laser emission threshold, generated by said first current supply means and said second current supply means; and a plurality of switching means for connecting said third current supply means to said plurality of laser emitting means each so as to be freely switched on and off.
Accordingly, the third current supply means for generating the current for canceling out the current near the laser emission threshold, generated by the first current supply means and the second current supply means, is connected to the plural laser emitting means by the plural switching means each so as to be freely turned on and off, whereby the supply of current to the laser emitting means not driven is stopped by the current control means. Namely, the supply of the current near the laser emission threshold to the laser emitting means is turned on and off at high speed without turning on and off the connection of the laser emitting means to the first current supply means and the second current supply means.
The present invention also provides the above-stated laser driving apparatus wherein the first current supply means are comprised of slow starter power supplies. Therefore, the first current supply means do not generate too high inrush currents as pulse currents, whereby the laser emitting means can be protected well. Since the connection is not turned on and off between the laser emitting means and the first current supply means in order to turn on and off the supply of the current near the laser emission threshold to the laser emitting means at high speed, the first current supply means can supply the pulse current without a delay even if they are comprised of the slow starter power supplies.
An image-forming apparatus of the present invention comprises the laser driving apparatus described above, data supply means for supplying image data as the control signal to said signal input means; beam deflecting means for deflecting the plural laser beams emitted from said laser driving apparatus to effect scanning in a main scanning direction according to the control signal supplied from said data supply means; latent image carrying means arranged to be exposed to and scanned with the plural laser beams under deflection scanning carried out by said beam deflecting means; sub-scanning means for moving said latent image carrying means relative to said beam deflecting means in a sub-scanning direction; latent image developing means for developing a latent image formed on said latent image carrying means with toner; toner transferring means for transferring the toner on said latent image carrying means, developed by said latent image developing means, onto a recording medium; and toner fixing means for fixing the toner transferred onto the recording medium by said toner transferring means.
In the image-forming apparatus of the present invention, therefore, the data supply means supplies the image data as a control signal to the signal input means of the laser driving apparatus of the present invention, so that the laser emitting means of the laser driving apparatus emit the respective laser beams controlled according to the image data. These plural laser beams are deflected by the beam deflecting means to scan in the main scanning direction, the latent image carrying means is exposed to and scanned with the laser beams thus deflected to scan, and the latent image carrying means thus exposed and scanned is moved relative to the beam deflecting means in the sub-scanning direction by the sub-scanning means. The latent image thus formed on the latent image carrying means is developed with the toner by the latent image developing means and the toner thus developed on the latent image carrying means is transferred onto the recording medium by the toner transferring means. The toner transferred onto the recording medium is fixed by the toner fixing means, so that the image corresponding to the image data is formed as a toner image on the recording medium by the electrophotographic method. In the image-forming apparatus of the present invention utilizing the laser driving apparatus of the present invention, the image is formed while the laser beams are controlled according to the image data. Since a plurality of main scanning lines are exposed simultaneously with the plural laser beams, the image of a dot matrix is formed at high speed. The image is formed with high quality, because the emitted light amounts of the plural laser beams are adjusted each properly.
The various means stated in the present invention can be any means that can implement their functions; for example, they permit dedicated hardware, a computer provided with the appropriate functions in the form of programs, functions implemented inside a computer by appropriate programs, a combination of these, and so on.